The invention relates to a mass spectrometer, more particularly for simultaneously measuring beams of a number of species of ions, having a special homogeneous magnetic sector field, the exit boundary of said field forming a straight line which extends through the point of inersection between the central ray of the incident object-ray pencil of ions and the straight entrance boundary of said field, at least one of the emergent image-ray pencils of ions undergoing second-order directional focusing.
In a known mass spectrometer of this kind (German Patent Specification 140 748) the entrance boundary and the exit boundary of the sector field form a straight, common boundary line. The entrance slit of the object-ray pencil of ions lies on the entrance boundary of the sector field. In the known mass spectrometer (German Specification 1 140 748), in order to reduce the second-order spherical aberration of conventional 180.degree. spectrometers, in which the central ray of the object-ray pencil extends into the entrance slit at right angles to the entrance boundary of the sector field, i.e. at a zero angle .epsilon..sub.1 to the line perpendicular to the entrance boundary, and wherein the exit boundary of the exit field coincides with the straight image line containing the image points of the individual image-ray pencils separated by mass, the included angle .epsilon..sub.1 is made different from zero, i.e. is chosen between -54.degree. and 0.degree., the straight image line forming an acute angle with the sector-field boundary. The included angle is negative if, when regarded from the optical axis, it lies on the side towards the centre points of the arcs through which the individual species of ions are deflected; it is positive if it lies on the other side of the optical axis. In the known spectrometers there is only one included angle, i.e. .epsilon..sub.1 = - 35.degree.16', at which the second-order spherical aberration is zero, i.e. where second-order directional focusing occurs. In the aforementioned spectrometers, the lateral magnification V is equal to unity, thus greatly limiting their resolving power, i.e. their ability adequately to separate ions having higher mass numbers. Since the angle .epsilon..sub.1 is negative, the object-ray pencil in the boundary field of the magnetic sector field is scattered in a direction perpendicular to the deflecting plane of the sector field, thus resulting in considerable losses of intensity.